plantago major
薘 refers specifically to the plant Plantago major, commonly known as broadleaf plantain. It is a rare kanji with a single, narrow botanical meaning.
薘 is a phono-semantic compound: the grass radical 艹 indicates a plant, while 達 likely contributes the sound. The exact historical development is uncertain.
The grass radical 艹 sits above 達, like a plant that reaches (達) upward. Picture a broadleaf plantain growing tall and reaching for the sun.
For タツ, imagine a plantain leaf with a small tack stuck in it: tack -> タツ, and the leaf is from the 薘 plant.